This invention relates to an active noise cancellation apparatus in an MRI apparatus. More in particular, it relates to an active noise cancellation apparatus for actively cancelling noise near the ears of a patient.
When a pulsating current is caused to flow through a gradient magnetic field coil fixed directly onto a bobbin in a conventional MRI apparatus, the bobbin is vibrated by an electromagnetic force and a big noise occurs from the bobbin as a whole and imparts an offensive feel to a patient. Even when a vibration-proofing rubber is sandwiched between the gradient magnetic field coil and the bobbin to proof the noise, the gradient magnetic field coil undergoes deformation to the extent corresponding to contraction of the vibration-proofing rubber between the coil and the bobbin due to the load, because of the pulsating load generated by the electromagnetic force. As a result, a change occurs in the magnetic field distribution inside a bore, which is precisely generated by the gradient magnetic field coil, and distortion of an image different from an actual image occurs in the resulting image. In this case, the softer the vibration-proofing rubber, the higher becomes the vibration-proofing effect but the greater becomes the strain of the coil. Consequently, distortion of the image becomes greater, as well.
The application of active noise cancellation technique has been investigated to cope with the problems described above, and the following references are known as the related art to the present invention.
(1) International Publication No. WO 90/02513
This reference introduces the technique which collects sound through microphones disposed close to the ears of a patient inside an MRI apparatus, generates sound from speakers and executes a control so that the sound collection quantity from the microphones becomes small.
Although this method provides the noise cancellation effect when the same noise waveforms are repeated, it becomes ineffective against random noises.
Since the noise itself inside the bobbin is detected as a noise source signal by the microphones, the sound waves generated from the speakers are picked up by the microphones and the microphone output signals are in turn fed back to the sound waves generated by the speakers. Accordingly, howling develops and active noise cancellation control cannot be carried out stably.
(2) PCT Japan Laid-Open No. Hei 1-501344 (International Laid-Open No. WO 88/02912)
This reference discloses a technique which actively cancels a sound generated by a machine, etc. More definitely, this method detects a rotation signal of a rotary shaft as data of a vibration source of a machine, and decreases signal energy of a sound sensor placed at a position at which a noise is desired to be cancelled. However, this technique involves the following problems. The noise to be cancelled comprises mainly periodic components having a number-of-revolution component as a fundamental component, and the noise to be lowered is limited to selected harmonics. Accordingly, when the noise contains random components, it cannot be cancelled because an original reference signal does not contain the random components.
When the number-of-revolution is extremely high, even if a sound is generated from a noise cancellation sound source in order to reduce a sensor output at the position where noise cancellation is to be made by the use of the reference signal, the sound generated from the sound source causes a time delay due to a calculation time, etc., in an active noise cancellation apparatus, and comes to posses difference properties from the noise arriving from the noise source. Accordingly, there are the cases where noise cancellation cannot be made.
(3) JP-A-2-70195
This reference discloses the technique which disposes a microphone on a noise generation source side and another microphone on the downstream side in a noise propagation direction, generates a noise cancellation signal from the signals from these microphones, and cancels the noise by generating a sound from speakers disposed at intermediate portions of these two microphones in order to cancel the noise generated inside a duct of an air conditioning equipment, and so forth.
However, this technique cannot be applied to the case where the noises propagate from all directions, although it can be applied to the case where the noises propagate from only one direction.